Question: Which of these numbers is prime? ${6,\ 33,\ 55,\ 59,\ 81}$
A prime number has exactly two factors: 1 and itself. The factors of 6 are 1, 2, 3, and 6. The factors of 33 are 1, 3, 11, and 33. The factors of 55 are 1, 5, 11, and 55. The factors of 59 are 1 and 59. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 59 is a prime number.